A blog on consciousness by Janet Kwasniak

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The thalamus as conductor

We might think that the thalamus controls the input to the cortex and then is involved in consciousness, attention and working memory, but, it is the cortex alone that does the cognitive work. Not so fast. Two 2009 studies by M Sherman’s group in Chicago (here) show that the thalamus stays involved.

The first paper:

One set of experiments, conducted by Brian Theyel and Daniel Llano in Sherman’s laboratory and published online December 6 (2009) in Nature Neuroscience The flavoprotein autofluorescence imaging technique, developed with University of Chicago assistant professor of neurobiology Naoum Issa, allowed the researchers to observe neuronal activity in a specially-prepared mouse brain slice that preserved connections between thalamus and somatosensory cortex. Once sensory information reaches the cortex, it is thought to remain segregated there as it moves from primary cortex to secondary cortex and higher-order areas. But when Theyel severed the direct connection between primary and secondary cortical regions, stimulating primary somatosensory cortex still activated secondary cortex as well as the thalamus, suggesting a robust pathway from cortex to thalamus and back. Only when the thalamus itself is interrupted does the activation of secondary cortex fail. The observation that at least a portion of sensory information passes back through the thalamus on its travels between cortical areas refutes the notion of the thalamus as a passive, one-time relay station, Theyel and Sherman said. “The ultimate reality is that without thalamus, the cortex is useless… The somatosensory pathway finding demonstrates for the first time that this corticothalamocortical loop, which is also present in the auditory and visual systems, significantly activates cortex. Keeping the thalamus “in the loop” may help the brain coordinate sensory information with motor systems to direct attention or coordinate multiple cortical areas to accomplish different tasks, Sherman said. “The thalamus is a remarkable bottleneck,” Sherman said. “But that may be because as a bottleneck, it provides a convenient way to control the flow of information. It is a very strategically organized structure.”

The second paper:

In the PNAS paper, published online on December 7, postdoctoral researcher Charles Lee mapped two auditory pathways entering different parts of the thalamus to see whether they carried the same or different information….Lee recorded from neurons in different areas of the thalamus while stimulating different areas of the inferior colliculus, another brain region of the auditory pathway. When the central nucleus of the inferior colliculus was stimulated it excited an area in the thalamus known to project to primary auditory cortex, suggesting that this is the direct route for auditory information through the brain. …By contrast, stimulating the surrounding “shell” region of the inferior colliculus provokes a different response, sending a mixed combination of excitatory and inhibitory input to a different region of the thalamus in contact with higher-order cortex. “These are two parallel streams serving different functions,” Lee said. “The thalamus is also the central hub for transferring information between cortical areas. Rather than carrying information, this second pathway winds up modulating information being sent between cortical areas.”

Summing up:

(The thalamus is) not a crossroads, but a conductor. “These experiments not only give you a new way of looking at how cortex functions, but also answers a question about what most of thalamus is doing,” Sherman said. “People who study how the cortex functions now have to take the thalamus into account. This can’t be ignored.”